Many of the monomers having an unsaturated bond with reaction activity can yield a polymer by selecting a catalyst that can cause cleavage of the unsaturated bond to initiate a chain reaction and appropriate reaction conditions. In general, there are an extremely wide variety of kinds of the monomers having an unsaturated bond, so that kinds of the resins that can be obtained therefrom also come in a great many varieties. However, there are relatively few kinds of monomers that can yield a product having a high molecular weight of 10,000 or higher, which is generally called as a high molecular compound. For example, typical monomers include ethylene, substituted ethylenes, propylene, substituted propylenes, styrene, alkylstyrenes, alkoxystyrenes, norbornene, various acrylic esters, butadiene, cyclopentadiene, dicyclopentadiene, isoprene, maleic anhydride, maleimide, fumarate esters, allyl compounds, and the like. Various kinds of resins are synthesized from these monomers or various combinations thereof.
The use of these resins is mainly limited to the field of relatively inexpensive commercial equipment, and there is little application to the field of high technology such as semiconductor-related materials and the like. This is because heat resistance, low hygroscopicity and permittivity have not been simultaneously achieved.
For instance, in the field of semiconductor-related materials, on account of increased density in integration in recent years, it has been desired to further attain low permittivity in addition to heat resistance and low hygroscopicity that have already been achieved. In order to achieve low permittivity, it is indispensable in principle to decrease the number of polar groups in a resin. At present, polyimides are often employed as resins for semiconductors. However, a lot of hard work has been made to achieve low permittivity because polyimides contain many carbonyl groups in a resin skeleton. As measures to overcome the situation, researches using the monomers containing fluorine have been done extensively, but sufficiently low permittivity has not been achieved. Moreover, there are some problems such as the rising price of resins, complicated synthesis of resins, and the like.
As another method, attempts to synthesize a polymer comprising a hydrocarbon containing no polar groups have been made. An example of such a polymer is a series of polymers called cyclic polyolefins. Specific examples include a polymer obtained by hydrogenating polynorbornene, or a polymer comprising polydicyclopentene and a derivative thereof. Although these polymers can exhibit extremely low permittivity, they have problems such as low heat resistance and very high permeability of water in spite of extremely low water absorption. In particular, the high permeability of water is a common characteristic of polyolefins, and it is considered to be extremely difficult to solve this problem.
Another example is a syndiotactic polystyrene synthesized by using Ziegler-Natta catalyst or Kaminsky catalysts. This polymer has a structure that three-dimensional positions of the benzene rings to the backbone are located alternately in the opposite directions, so that it is possible to attain very high heat resistance and at the same time extremely low water absorption, extremely low permeability of water and very low level of permittivity. However, this polymer has such high crystallinity that it has a problem of considerably poor adhesion to a base material and also has another problem that methods of processing it are markedly limited because it is insoluble in any solvents. That is, at present, a polymer that can overcome the above-mentioned problems has not been developed yet.
On the other hand, typical polymers for optical uses, such as optical lenses, optical waveguide materials and the like include acrylic resins and polyolefin resins. Acrylic resins have characteristics of having excellent transparency and workability and extremely low birefringence. However, they have disadvantages that they have high hygroscopicity, relatively low heat resistance and low toughness. By contrast, polyolefin resins have excellent heat resistance and extremely low hygroscopicity, but they are inferior to acrylic resins in transparency and low birefringence. That is, both of acrylic resins and polyolefin resins have both advantages and disadvantages, and thus it has strongly been desired to develop a resin compensating for the disadvantages of acrylic resins and polyolefin resins.
Thus, in order to improve acrylic resins, that is, to overcome the disadvantages such as high hygroscopicity and low heat resistance, a lot of investigations have been carried out. For example, there is a method of improving hygroscopicity and heat resistance by using a monomer having a bulky substituent (disclosed in Japanese Patent No. 2678029). Although this invention is indeed effective to a certain extent, the improved acrylic resin is still interior to polyolefin resins in hygroscopicity. This invention poses still another problem that there is a drastic reduction in toughness and strength because a bulky substituent is present in a side chain, so that the resin becomes likely to be broken particularly in molding processing. Although there is a method of attempting to give toughness to the resin by copolymerizing a monomer that gives flexibility for the purpose of improving this, decrease in heat resistance is inevitable, and thus the effect of introducing a bulky substituent is weakened.
Polyolefin resins have extremely great advantages of low hygroscopicity and high heat resistance as resins of optical use, but the high birefringence thereof has become a great disadvantage with the increasing sophistication of optical devices in recent years. Therefore, many attempts to lower the birefringence of polyolefin resins have been made particularly recently.
Such an example is disclosed in Japanese Patent Application Laid-open No. Hei 8-110402. This invention is that a resin or a low molecular weight compound having birefringence with the opposite sign to the birefringence of a polyolefin resin is mixed to the polyolefin resin to compensate the birefringence intrinsic to the resin, thereby reducing the birefringence of the resin mixture to zero. In this method, it is required that a resin to be mixed and a polyolefin resin be completely compatible. However, compatibility of a polyolefin resin and a resin that is claimed is insufficient in the above invention, so that satisfactory effect cannot be achieved.
Thus, in order to realize as complete compatibility as possible, a method of adding a compatibility agent as the third component is carried out as a polymer alloying technique, and it is specifically described in U.S. Pat. No. 4,373,065. In order to mix both of the above highly homogeneously, both should be in a molten state or a solution state. However, it is considered that it is extremely difficult to obtain a practical polymer material that is highly homogeneous and has no birefringence as a whole by using any physical method.
A method to solve these problems is disclosed in Japanese Patent Application No. Hei 8-199901. This method has some problems such as remaining a portion of a resin in a die when removing a product from the die in injection molding of the resin composition or breaking a product in mold releasing. Moreover, it has a disadvantage in that the color of a product changes while the resin stays in a molding machine for a long time.